This invention relates to burners and particularly to premix gas burners used, for example, in boiler or other heat exchangers for heating and other systems.
Burners used in boiler or heat exchange systems, for example, have in the past included a main gas tube wherein gas is ignited by a direct spark. Such direct spark "ignition" is sensitive to ignition timing and to the flow and mix characteristics of the gas and air mixture. If the flow starts too far ahead of the spark, or if the flow is not homogeneous or not in the proper ratio, the light off of the burner flame can be exciting. The process can result in a delayed light off, causing damage or at least an annoying "boom" in the system. Moreover, when the ignitor is disposed within the main gas flame, it is constantly heated and is prone to increased deterioration.
In another aspect of gas burners, it is common to direct mixed gas and air through a burner plate which serves to define a flame front. It is desirable to fashion such plates from metal and to provide apertures therethrough for the gas and air mixture. In order to provide for efficient burning, it has been suggested to use a shaped metal sheet providing a plurality of gas passages therethrough. U.S. Pat. No. 4,397,631 illustrates one example of such a burner. In that patent, holes are provided in a thin metal member which is contoured. Such contour lends strength and rigidity to the burner plate when thin metal is used. Without strengthening contours, thin burner plates tend to warp and distort in the presence of the burner heat.
When providing a burner plate, it is desirable to use a plurality of gas and air apertures therein. Problems are presented, however, as a function of hole size to metal thickness when combined with use in a very hot environment. For example, where thin metal is used, the heat generated by the burner flame can cause the metal burner plate to warp and distort, adversely affecting the flame front. On the other hand, where thicker metals are used, the ability to provide holes of smaller diameter than the metal thickness is compromised. Specifically, as metal thickness increases, it becomes more difficult to punch a hole of a given diameter smaller than that thickness. As a rule of thumb, where the hole diameter is smaller than the metal thickness, punches break more often and tooling expense in manufacturing increases. Nevertheless, a number of holes of or below a certain diameter are required on the plate area to pass the gas and air mixture properly and produce the desired flame front. A designer is thus caught between the need to use a thick burner plate for rigidity and the need to use small diameter holes to produce a stable flame front. Accordingly, it is difficult to provide the necessary cross-sectional gas and air flow areas through a burner plate in a material thick enough to prevent warpage and distortion in the presence of a burner flame and at the same time produce a desired flame front without flashback.
It is thus one objective of this invention to provide an improved ignition apparatus for a gas-fired burner and to avoid the direct spark light off of the main burner flame.
Another objective of this invention has been to provide an improved burner plate with sufficient gas and air flow, yet sufficiently strong enough to withstand warpage distortion and to prevent flashback when used proximate a burner flame.
In another but related aspect of gas burners, it is important to produce a stable flame front which does not jump or migrate across the burner face at an audible resonance sounding like a high squeal. This can occur when the gas and air flow holes are too big.
Another objective of the invention has thus been to quiet gas burners during operation.
To these ends, a preferred embodiment of the invention comprises a premix gas burner having a blower for forcing air through the burner, a burner tube for receiving gas and air, a pilot tube extending through the burner tube for conveying pilot gas to a burner end of the burner tube and supporting two members of a composite burner plate. An ignitor extends through the composite burner plate for igniting gas flowing through the pilot tube to produce a pilot flame just outside the burner plate. When the main gas is started, the pilot flame ignites it easily, without damage or the `boom` associated with a direct spark ignitor.
The ignitor itself includes a center electrode encased in an insulating sleeve, and another electrode wrapped spirally around the sleeve and extending forwardly with, but spaced from, the center electrode. When the ignitor is assembled in the pilot tube, the outer surfaces of the wrapped electrode engage the inner surface of the pilot tube, forming an electrical ground. On application of current to the center electrode, a pilot spark is created across the tips of the two electrodes to ignite a pilot flame. After the pilot is lit, the main gas and air flow is started and is ignited softly by the pilot flame. Since the electrodes remain in the rich gas and air flow of the pilot tube, such flow tends to cool them even when the main flame is lit.
In another aspect of the invention, the burner plate is a composite of two flat component plates. The outer or forward component plate has a plurality of small holes while the inner or rearward component plate has a plurality of larger holes. The two plates are welded together so each larger aperture in the inner plate overlies a plurality of smaller apertures in the outer plate. The matrix between the larger apertures in the inner plate blocks underlying smaller apertures and provides a surface for spot welding the plates together. Gas and air mixture flows from inside the burner tube through the larger holes and then through the smaller apertures to outside the composite plate where the mixture is ignited by the pilot flame and then the main gas flame in a flame front.
The inner and out component plates are each individually relatively thin, thus accommodating holes in each of a diameter in excess of each respective plate thickness. The thinness of the plates accommodates the desirable hole sizes. Yet when the plates are welded together, they form a composite, flat burner plate sufficiently strong enough to withstand heat of the burner without warpage or distortion, even though the composite plate is flat, and without strengthening contours. The composite plate also provides appropriate flow apertures preventing flashback and flame front migration, quieting the burner operation.
Moreover, since the plates are captured on the pilot tube, the entire pilot tube and burner plate can be easily removed and serviced from the rear of the burner, it being unnecessary to disassemble the entire burner from its environment for service.